Devices, structures, materials and methods for vertical light emitting transistors and light emitting displays

ABSTRACT

Devices, structures, materials and methods for vertical light emitting transistors (VLETs) and light emitting displays (LEDs) are provided. In particular, architectures for vertical polymer light emitting transistors (VPLETs) for active matrix organic light emitting displays (AMOLEDs) and AMOLEDs incorporating such VPLETs are described. Porous conductive transparent electrodes (such as from nanowires (NW)) alone or in combination with conjugated light emitting polymers (LEPs) and dielectric materials are utilized in forming organic light emitting transistors (OLETs). Combinations of thin films of ionic gels, LEDs, porous conductive electrodes and relevant substrates and gates are utilized to construct LETs, including singly and doubly gated VPLETs. In addition, printing processes are utilized to deposit layers of one or more of porous conductive electrodes, LEDs, and dielectric materials on various substrates to construct LETs, including singly and doubly gated VPLETs.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/550,656 filed Nov. 21, 2014, which application claims priority toU.S. Provisional Application No. 61/907,324 filed Nov. 21, 2013, thedisclosures of which are incorporated herein by reference in theirentireties.

FIELD OF THE INVENTION

Devices, structures, materials and methods for vertical light emittingtransistors and light emitting displays are presented.

BACKGROUND OF THE INVENTION

Active Matrix Organic Light Emitting Diode (AMOLED) displays have beenhighly anticipated by electronic consumers due to their well-recognizedadvantages in power consumption, pixel brightness, viewing angle,response time, and contrast ratio over liquid crystal displays (LCD).(See, e.g., G. Gu and S. R. Forrest, IEEE Journal of Selected Topics inQuantum Electronics, vol. 4, pp. 83-99, 1998, the disclosure of which isincorporated herein by reference.) However, the advances promised byAMOLEDs have not been realized at least partially as the result oflimitations in conventional active matrix thin-film transistor (TFT)backplanes. The current active matrix TFT backplanes used to driveAM-LCD pixels are typically made of amorphous silicon (a-Si), which hasa low mobility (−1 cm²V⁻¹s⁻¹) and poor stability, and is thereforeunsuitable for AMOLED pixels. (See, M. J. Powell, IEEE Transactions onElectron Devices, vol. 36, pp. 2753-2763, 1989, the disclosure of whichis incorporated herein by reference.) As a result of these deficiencies,currently AMOLED displays are driven by low temperature polycrystallinesilicon (poly-Si) TFTs that suffer from high fabrication cost and time,and device size, orientation, and inhomogeneity limitations, all ofwhich present a severe challenge to increasing display size andproduction yield. (See, e.g., C.-P. Chang and Y.-C. S. Wu, IEEE electrondevice letters, vol. 30, pp. 130-132, 2009; Y.-J. Park, M.-H. Jung,S.-H. Park and O. Kim, Japanese Journal of Applied Physics, vol. 49, pp.03CD01, 2010; and P.-S. Lin, and T.-S. Li, IEEE electron device letters,vol. 15, pp. 138-139, 1994, each of the disclosures of which areincorporated herein by reference.)

Solution processible organic semiconductor materials are attractivealternatives to poly-Si because of their homogeneity, low cost, andvaried deposition methods. (See, e.g., D. J. Gundlach, et al., IEEEElectron Device Letters, vol. 18, pp. 87-89, 1997; H. Yan, et al.,Nature, vol. 457, pp. 679-686, 2009; and A. L. Briseno, et al., Nature,vol. 444, pp. 913-917, 2006, the disclosures of each of which areincorporated herein by reference.) However, in a conventional TFTarchitecture, the low-mobility of organic films requires a largesource-drain voltage (>20 V) to turn on the OLED devices. (See, H.Sirringhaus, et al., Science, vol. 280, pp. 1741-1744, 1998, thedisclosure of which is incorporated herein by reference.) Stablehigh-transconductance organic thin-film electrochemical transistorsusing a high capacitance electrolyte as the gate dielectric layer havebeen demonstrated. (See, e.g., J. H. Cho et al., Nature Materials, vol.7, pp. 900-906, 2008; and Y. Xia, et al., Advanced Functional Materials,vol. 20, pp. 587-594, 2010, the disclosures of each of which areincorporated herein by reference.) Using these devices it is possible tocontrol a high efficiency red, green and blue AMOLED with supplyvoltages near 4 V and sub-1 V driving voltages. (See, e.g., D. Braga, etal., Advanced Functional Materials, vol. 22, pp. 1623-1631, 2012, thedisclosure of which is incorporated herein by reference.) In addition,carbon nanotube enabled vertical organic thin-film field effecttransistors that give on-currents sufficient to drive OLED pixels at lowoperating voltages because of their intrinsic short channel lengths havealso been demonstrated. (See, e.g., M. A. McCarthy, et al., Science,vol. 332, pp. 570-573, 2011, the disclosure of which is incorporatedherein by reference.) Though these approaches are promising, therequired fabrication steps still limit the simplicity of systemarchitecture and consequently production costs.

SUMMARY OF THE INVENTION

Devices, structures, materials and methods for vertical light emittingtransistors (LETs) and light emitting displays (LEDs) are provided.

Some embodiments are direct to vertical light emitting transistors,including:

-   -   a light emitting cell including a light emitting layer formed of        at least one light emitting material, the light emitting layer        having first and second sides in conductive relation to a        conductive drain electrode and a conductive source electrode;    -   at least one capacitor including a dielectric layer formed of at        least one dielectric material, the at least one dielectric layer        having first and second sides in conductive relation to one of        either the conductive source or drain electrodes, and a        conductive gate electrode; and    -   at least one substrate in supportive relation with each of said        drain/source and gate electrodes;    -   wherein the drain and source electrodes are the cathode and        anode of the light emitting cell; and    -   wherein at least the electrode disposed between the light        emitting layer and the dielectric layer is a conductive porous        electrode has sufficient open portions to exhibit a surface        coverage of no greater than 50%, such that the dielectric layer        makes direct contact with the light emitting layer through the        open portions of the conductive porous electrode.

In other embodiments the vertical light emitting transistor includes:

-   -   at least two capacitors and two gate electrodes, a first        capacitor having a dielectric layer disposed between the drain        electrode and a first gate electrode and a second capacitor        disposed between the source electrode and a second gate        electrode; and    -   wherein the drain and source electrodes are conductive porous        electrodes that have sufficient open portions to exhibit a        surface coverage of no greater than 50%, such that both of the        dielectric layers make direct contact with the light emitting        layer through the open portions of the conductive porous        electrodes.

In still other embodiments, the light emitting layer is formed of alight emitting material selected from the group consisting of acrystalline semiconductor selected from GaN, GaP, GaAs, AlGaAs, GaAsP,AIGaInP, ZnSe, InGaN and AIN; a semiconductor nanowire selected from Siand GaAs; a quantum wall; an organometallic complex; an Irorganometallic complex; a small organic conjugated molecule; porphyrin;pentacene; and a conjugated polymer selected from PPV, PVK, MEH-PPV,PPF, PFO and PPP.

In yet other embodiments, at least one of the drain, source, or gateelectrodes comprise an electrode material selected from graphene sheets,doped Si, ZTO, ITO, Au, Al, Cu, Ni, Mo, Cr, Ag, metal nanowires, metalplate, metal meshes, metal grids, holey copper, holey graphene,conductive polymers, and a low coverage network of a plurality ofnanowires.

In still yet other embodiments, the at least one conductive porouselectrode is formed from a plurality of nanowires formed into one of thegroup of a random or patterned network of a plurality of metal orgraphene nanowires, a nanowire metal mesh, a nanowire grid, and ananowire network encased within an elastomeric material. In some suchembodiments the plurality of nanowires are formed from a plurality ofmetal nanowires selected from Ag, Au and Cu having an aspect ratio of atleast 1000. In other such embodiments the plurality of metal nanowireshave a diameter less than about 200 nm and a length greater than about 1micron, and having a surface coverage less than 10%, a sheet resistanceless than 100 Ω/sq and a transmission greater than 75%.

In still yet other embodiments, the dielectric material is selected fromthe group of an oxides selected from SiO₂, Al₂O₃, HfO₂, ZrO₂; a nitride;Si₃N₄; an inorganic salts selected from LiF, CsF, BaTiO₃, and SrTiO₃; adielectric polymer selected from PMMA, Teflon, CYTOP, and Nafion; and anionic gel formed from the combination of a dielectric polymer and anionic liquid.

In still yet other embodiments, the vertical light emitting transistorsmay include at least one additional light emitting enhancement layerselected from the group consisting of electron injection dipole layers,transportation dipole layers, conjugate polyelectrolyte layers, and holeinjection layers.

In still yet other embodiments the substrate is selected from the groupconsisting of flexible plastics, Si wafer, glass, sapphire, and ITO.

Other embodiments are directed to methods of forming vertical lightemitting transistors, including:

-   -   forming a light emitting layer formed of at least one light        emitting material, the light emitting layer having first and        second sides;    -   forming at least one of a conductive drain electrode and a        conductive source electrode in conductive relation with at least        one side of said light emitting layer;    -   forming a least one capacitor including a dielectric layer        formed of at least one dielectric material, the at least one        dielectric layer having first and second sides in conductive        relation to one of either the conductive source or drain        electrodes, and a conductive gate electrode; and    -   encapsulating the formed layers using one of either a glass or        barrier film;    -   wherein at least the electrode disposed between the light        emitting layer and the dielectric layer is a conductive porous        electrode that has sufficient open portions to exhibit a surface        coverage of no greater than 50%, such that the dielectric layer        makes direct contact with the light emitting layer through the        open portions of the conductive porous electrode.

In other embodiments the method includes one of the followingcombination of steps:

-   -   disposing the light emitting material atop a PEDOT coated ITO        substrate, disposing one of either the drain or source electrode        atop the light emitting layer, disposing the dielectric layer        atop the one of either the drain or source electrode present        atop the light emitting layer, and disposing the gate electrode        atop the dielectric layer;    -   disposing a first dielectric layer atop a first gate electrode,        disposing one of either the drain or source electrode atop the        first dielectric layer, disposing the light emitting layer atop        the one of either the drain or source electrode disposed atop        the first dielectric layer, disposing one of either the drain or        source electrode atop the light emitting layer; disposing a        second dielectric layer over the one of either the drain or        source electrode disposed atop the light emitting layer, and        disposing a second gate electrode atop the second dielectric        layer;    -   disposing the dielectric layer atop the gate electrode,        disposing one of either the drain or source electrode atop the        dielectric layer, and disposing the light emitting layer atop a        PEDOT coated ITO substrate that is further laminated with the        one of either the drain or source electrode which is disposed        atop the dielectric layer; and    -   disposing the dielectric layer atop the gate electrode,        disposing one of either the drain or source electrode disposed        atop the dielectric layer, disposing the light emitting layer        atop the one of either the drain or source electrode disposed        atop the dielectric layer, and disposing the light emitting        layer that is laminated with the one of either the drain or        source electrode not disposed atop the dielectric layer.

In still other embodiments, the light emitting layer is formed by adeposition process selected from plasma enhanced chemical vapordeposition, low pressure chemical vapor deposition, molecular beamepitaxy, spin-coating, spray coating, aerosol jet printing, inkjetprinting, screen printing, gravure printing, and flexograph printing. Insome such embodiments the method further includes thermal annealing thelight emitting material in an inert atmosphere at a temperature range offrom 60 to 140° C. for a duration of about 10 to 60 minutes. In stillother such embodiments the light emitting layer is a conjugated polymerformed by spin-coating the conjugated polymer in an organic solutionhaving a concentration of 1 to 10 mg/mL with a speed range from 800 to5000 rpm followed by thermal annealing the conjugated polymer in aninert atmosphere at a temperature range of from 60 to 140° C. for aduration of about 10 to 60 minutes. In yet other such embodiments thelight emitting layer is a conjugated polymer formed by aerosol jetprinting the conjugated polymer in an organic solution using a sheathgas flow of about 20 to 50 cubic centimeters per minute, a carrier gasflow of about 10 to 20 cubic centimeters per minute, a nozzle diameterof from about 60 to 300 μm, and an atomization selected from ultrasonicatomization at a voltage that ranges from about 20 to 48 V, andpneumatic atomization with an atomizer flow of about 600 cubiccentimeters per minute, to generate an aerosol having a diameter ofabout 1 to 5 μm.

In yet other embodiments at least one of the source, drain, and gateelectrodes comprise a metal selected from Al, Au, Ag, Cu, Ni, Cr, Mo,and their combination formed by plasma enhanced chemical vapordeposition or low pressure chemical vapor deposition.

In still yet other embodiments the at least one conductive porouselectrode is formed from a plurality of nanowires formed into one of thegroup of a random or patterned network of a plurality of metal orgraphene nanowires, a nanowire metal mesh, a nanowire grid, a nanowirenetwork encased within an elastomeric material, holey copper and holeygraphene. In some such embodiments the conductive porous electrodecomprises a silver nanowire network with a surface coverage less than10%, a sheet resistance less than 15 Ω/sq, and a transmission greaterthan 75%, and is formed by a process selected from spin-coating, spraycoating, aerosol jet printing, inkjet printing, screen printing, gravureprinting, and flexography printing the silver nanowire network followedby thermal annealing the silver nanowire network in an inert environmentat a temperature range of from about 60 to 160° C. for a duration ofabout 10 to 60 minutes, and washing the silver nanowire network withisopropanol. In other such embodiments the conductive porous electrodeis formed by spin-coating a Ag nanowire isopropanol solution with aconcentration range of from about 0.1 mg/mL to 10 mg/mL with a spinningspeed range of from about 800 to 5000 rpm, and repeating saidspin-coating from about 1 to 20 times. In still other such embodimentsthe conductive porous electrode is formed by aerosol jet printing a Agnanowire isopropanol solution with a concentration range of from about0.1 mg/mL to 10 mg/mL using a sheath gas flow of about 20 to 50 cubiccentimeters per minute, a carrier gas flow of about 10 to 20 cubiccentimeters per minute, a nozzle diameter of about 60 to 300 μm,fiducial management with an overlay registration of about 1 to 2 μm, anda pneumatic atomization with about a 600 cubic centimeters per minuteatomizer flow to generate an aerosol having diameter of from about 1 to5 μm. In yet other such embodiments the porous electrode is one of ametal mesh, metal grid, holey copper and holey graphene formed by amethod selected from evaporating metal using a metal frame mask, andpatterning the metal using photolithography.

In still yet other embodiments the dielectric layer comprises adielectric material selected from SiO₂, Al₂O₃, Si₃N₄, HfO₂, ZrO₂ and LiFhaving a thickness of from about 50 to 500 nm formed by a processselected from one of either plasma enhanced chemical vapor depositionand low pressure chemical vapor deposition.

In still yet other embodiments the dielectrics layer comprises one of adielectric polymer, an ionic gel, and a dielectric polymer/nanoparticlecomposite having a thickness of from about 100 to 5000 nm formed by aprocess selected from spin-coating, spray coating, aerosol jet printing,inkjet printing, screen printing, gravure printing, and flexographprinting the dielectric material followed by thermally annealing thedielectric layer in an inert atmosphere at temperature range from about60 to 160° C. for a duration of from about 10 to 60 minutes.

In still yet other embodiments the dielectric layer is formed byspin-coating one of a dielectric polymer, an ionic gel, and a dielectricpolymer/nanoparticle composite disposed in an organic solution with aconcentration range of from about 0.1 mg/mL to 10 mg/mL at a spinningspeed range of from about 800 to 5000 rpm, and repeating for about 1 to20 times.

In still yet other embodiments the dielectric layer is formed by aerosoljet printing one of a dielectric polymer, an ionic gel, and a dielectricpolymer/nanoparticle composite in an organic solution having aconcentration range of from about 0.1 mg/mL to 10 mg/mL using apneumatic atomization with about 600 cubic centimeters per minuteatomizer flow or ultrasonic atomization with power range from 20-48V togenerate an aerosol in diameter of from about 1 to 5 μm, a sheath gasflow of from about 20 to 50 cubic centimeters per minute, a carrier gasflow of from about 10 to 20 cubic centimeters per minute, a nozzlediameter of from about 60 to 300 μm, and a fiducial management with anoverlay registration of from about 1 to 2 μm.

In still yet other embodiments the method includes forming at least twocapacitors and two gate electrodes, including a first capacitor having adielectric layer disposed between the drain electrode and a first gateelectrode and a second capacitor disposed between the source electrodeand a second gate electrode, wherein at least the drain and sourceelectrodes are conductive porous electrodes having a surface coverage ofno greater than 50%, such that both of the dielectric layers make directcontact with the light emitting layer through the open portions of theporous electrodes.

Still other embodiments are directed to vertical light emitting displaysincluding a plurality of pixels comprising a plurality of vertical lightemitting transistors electronically coupled into a thin film transistorbackplane, each vertical light emitting transistor including:

-   -   a light emitting cell including a light emitting layer formed of        at least one light emitting material, the light emitting layer        having first and second sides in conductive relation to a        conductive drain electrode and a conductive source electrode;    -   a least one capacitor including a dielectric layer formed of at        least one dielectric material, the at least one dielectric layer        having first and second sides in conductive relation to one of        either the conductive source or drain electrodes, and a        conductive gate electrode; and    -   at least one substrate in supportive relation with each of said        drain and gate electrodes;    -   wherein the drain and source electrodes are the cathode and        anode of the light emitting cell;    -   wherein at least the electrode disposed between the light        emitting layer and the dielectric layer is a conductive porous        electrode having sufficient open portions to exhibit a surface        coverage of no greater than 50%, such that the dielectric layer        makes direct contact with the light emitting layer through the        open portions of the conductive porous electrode;    -   wherein the light emitting layers emit one of either a single        color or a plurality of colors; and    -   wherein each pixel is formed of at least a first transistor        configured for switching and at least a second transistor        configured for scribing.

In other embodiments each of the vertical light emitting transistorsincludes at least two capacitors and two gate electrodes, a firstcapacitor having a dielectric layer disposed between the drain electrodeand a first gate electrode and a second capacitor disposed between thesource electrode and a second gate electrode, such that each pixel isformed of a single transistor configured for both switching andscribing, wherein the drain and source electrodes are conductive porouselectrodes that have sufficient open portions to exhibit a surfacecoverage of no greater than 50%, such that both of the dielectric layersmake direct contact with the light emitting layer through the openportions of the conductive porous electrodes.

In still other embodiments, the light emitting layer is formed of alight emitting material selected from the group consisting of acrystalline semiconductor selected from GaN, GaP, GaAs, AlGaAs, GaAsP,AIGaInP, ZnSe, InGaN and AIN; a semiconductor nanowire selected from Siand GaAs; a quantum wall; an organometallic complex; an Irorganometallic complex; a small organic conjugated molecule; porphyrin;pentacene; and a conjugated polymer selected from PPV, PVK, MEH-PPV,PPF, PFO and PPP.

In yet other embodiments, the dielectric material is selected from thegroup of an oxides selected from SiO₂, Al₂O₃, HfO₂, ZrO₂; a nitride;Si₃N₄; an inorganic salts selected from LiF, CsF, BaTiO₃, and SrTiO₃; adielectric polymer selected from PMMA, Teflon, CYTOP, and Nafion; and anionic gel formed from the combination of a dielectric polymer and anionic liquid.

In still yet other embodiments, the substrate is selected from the groupconsisting of flexible plastics, Si wafer, glass, sapphire, and ITO.

In still yet other embodiments, at least one of the drain, source, orgate electrodes comprise an electrode material selected from graphenesheets, doped Si, ZTO, ITO, Au, Al, Cu, Ni, Mo, Cr, Ag, metal nanowires,metal plate, metal meshes, metal grids, holey copper, holey graphene,conductive polymers, and a low coverage network of a plurality ofnanowires.

In still yet other embodiments, the at least one conductive porouselectrode is formed from a plurality of nanowires formed into one of thegroup of a random or patterned network of a plurality of metal orgraphene nanowires, a nanowire metal mesh, a nanowire grid, and ananowire network encased within an elastomeric material. In some suchembodiments the plurality of nanowires are formed from a plurality ofmetal nanowires selected from Ag, Au and Cu having a diameter less thanabout 200 nm and a length greater than about 1 micron, and having asurface coverage less than 10%, a sheet resistance less than 100 Ω/sqand a transmission greater than 75%.

Additional embodiments and features are set forth in part in thedescription that follows, and in part will become apparent to thoseskilled in the art upon examination of the specification or may belearned by the practice of the invention. A further understanding of thenature and advantages of the present invention may be realized byreference to the remaining portions of the specification and thedrawings, which forms a part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The description will be more fully understood with reference to thefollowing figures and data graphs, which are presented as exemplaryembodiments of the invention and should not be construed as a completerecitation of the scope of the invention, wherein:

FIG. 1 provides a schematic diagram of a vertical polymer light emittingtransistor in accordance with embodiments of the invention;

FIG. 2 provides an SEM image of an exemplary random silver nanowirenetwork in accordance with embodiments of the invention;

FIG. 3 provides a schematic diagram of a vertical polymer light emittingtransistor incorporating nanowire network electrodes in accordance withembodiments of the invention;

FIG. 4 provides a schematic diagram of a singly gated vertical polymerlight emitting transistor in accordance with embodiments of theinvention;

FIG. 5 provides a schematic diagram of a doubly gated vertical polymerlight emitting transistor in accordance with embodiments of theinvention;

FIGS. 6a and 6b provide circuit diagrams of many pixel containing lightemitting displays comprising a plurality of singly (6 a) and doubly (6b) gated vertical light emitting transistors in accordance withembodiments of the invention;

FIG. 7 provides a flow chart of a process for depositing verticalpolymer light emitting transistors in accordance with embodiments of theinvention;

FIG. 8 provides a schematic diagram of a Ag NW enabled vertical polymerlight emitting transistor in accordance with embodiments of theinvention;

FIG. 9a provides a schematic diagram of a Ag NW enabled vertical polymerlight emitting transistor with an ITO electrode on silicon wafer inaccordance with embodiments of the invention;

FIG. 9b provides an SEM of a Ag NW network on a Si wafer (left) and animage of the printed MEH-PPV polymer (right) of the vertical polymerlight emitting transistor of FIG. 9a in accordance with embodiments ofthe invention;

FIG. 10a provides a schematic diagram of a Ag NW enabled verticalpolymer light emitting transistor with a Ag NW/PET electrode on siliconwafer in accordance with embodiments of the invention;

FIG. 10b provides an SEM of a Ag NW/PET electrode of FIG. 10a inaccordance with embodiments of the invention;

FIG. 11 provides a schematic diagram of a singly-gated Ag NW enabledvertical polymer light emitting transistor with flexible electrodes, anda process diagram for its manufacture in accordance with embodiments ofthe invention;

FIG. 12 provides a schematic diagram of a doubly-gated Ag NW enabledvertical polymer light emitting transistor with flexible electrodes inaccordance with embodiments of the invention; and

FIG. 13 provides a schematic diagram of a VLET display on ITO inaccordance with embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Turning to the drawings, devices, materials and methods for lightemitting transistors (LETs) and light emitting displays (LEDs) areprovided. In particular embodiments, architectures for vertical lightemitting transistors (VLETs), and in particular vertical polymer lightemitting transistors (VPLETs) for active matrix organic light emittingdisplays (AMOLEDs) and AMOLEDs incorporating such VPLETs are described.Although many VLET architectures are provided, in embodiments a noveldoubly-gated VLET architecture is described. In other embodiments,porous transparent conductive electrodes, such as for example nanowire(NW) electrodes (including silver (Ag) NW networks) alone or incombination with conjugated light emitting polymers (LEPs) anddielectric materials are utilized in forming organic light emittingtransistors (OLETs). In some embodiments, combinations of thin films ofionic gels, LEDs, NW electrodes and relevant substrates and gates areutilized to construct LETs, including singly and doubly gated VLETs. Inmany embodiments the VPLET substrates are transparent and/or flexible,and can be made of suitable materials such as PET. In other embodiments,processes such as, for example, spin-coating and printing processes areutilized to deposit layers of one or more of NW electrodes, LEDs, anddielectric materials on various substrates to construct LETs, includingsingly and doubly gated VLETs.

Active matrix organic light emitting displays (AMOLEDs) are highlyattractive due to their power saving, ultra high definition, and broadviewing angles. In particular, advances in organic light emittingtransistors (OLETs) exhibit improved external efficiency over organiclight emitting diodes (OLEDs) by directly modulating charge carriers oflight emitting materials. Further, inducing a vertical structure inOLETs (as shown in FIG. 1) circumvents the intrinsic low mobility oforganic materials by providing short channel length, thereby making itpossible to achieve high conductance at low power and low voltages, thusenhancing the energy conversion efficiency, the lifetime and stabilityof the organic materials. Moreover, combining thin film transistor (TFT)switching and OLED light emitting properties in a single device leads toa simplified fabrication process and reduced cost. However, technicalchallenges in forming the underlying TFT backplanes in these deviceslimit display size variation and cost reduction. As will be describedbelow, the use of novel material and manufacturing combinations, such ashighly transparent porous conductive electrodes (e.g., NW networkelectrodes), and flexible materials enable the formation of VLETs thatovercome the limitations in display backplanes fabricated withamorphous/crystalline/poly silicon, metal oxides and organic materials,and will be suitable for various needs.

Transparent Porous Conductive Materials

As previously discussed, OLETs have been demonstrated to exhibit ON/OFFfunctionality of TFTs and the electroluminescence of OLEDs in a singledevice. (See, e.g., J. Zaumsei, R. H. Friend and H. Sirringhaus, Naturematerials, vol. 5, pp. 69-74, 2005; R. Capelli, et al., NatureMaterials, vol 9, pp. 496-503, 2010; and M. Muccini, Nature Materials,vol. 5, pp. 605-613, 2006, the disclosures of which are incorporatedherein by reference.) Vertical-type OLETs have been reported on varieddesigns and operating principles to operate in low-voltages with shortchannel length. In particular, LEPs have been fabricated into PLETs withnoble metals as electrodes. (See, e.g., J. Zaumsei, R. H. Friend and H.Sirringhaus, Nature materials, vol. 5, pp. 69-74, 2005, the disclosureof which is incorporated herein by reference.) Various VPLETs were alsoreported that demonstrated poor performance. (See, e.g., B. Park and H.Takezoe, Applied Physics Letters, vol. 85, pp. 1280-1282, 2004; H.Lechi, et al., Synthetic Metals, vol. 154, pp. 149-152, 2005; K. Kudo,Current Applied Physics, vol. 5, pp. 337-340, 2005; S. Y. Oh, et al.,Molecular Crystals and Liquid Crystals, vol. 458, pp. 247-254, 2006; Z.Xu, et al., Applied Physics Letters, vol. 91, pp. 092911, 2007; and H.Yamauchi, et al., Japanese Journal of Applied Physics, vol. 46, pp.2678, 2007, the disclosures of each of which are incorporated herein byreference.) In addition, severe material limitations exist as to usableelectrode materials. In particular, electrodes of transmittance greaterthan 98% across the visible spectrum window are required to allow forthe emission of light in these devices across their full aperture. (See,e.g., B. Park and H. Takezoe, Applied Physics Letters, vol. 85, pp.1280-1282, 2004; H. Lechi, et al., Synthetic Metals, vol. 154, pp.149-152, 2005; K. Kudo, Current Applied Physics, vol. 5, pp. 337-340,2005; S. Y. Oh, et al., Molecular Crystals and Liquid Crystals, vol.458, pp. 247-254, 2006; Z. Xu, et al., Applied Physics Letters, vol. 91,pp. 092911, 2007; H. Yamauchi, et al., Japanese Journal of AppliedPhysics, vol. 46, pp. 2678, 2007; and K. Nakamura, et al., JapaneseJournal of Applied Physics, vol. 47, pp. 1889, 2008, the disclosures ofeach of which are incorporated herein by reference.)

Transparent conductive electrodes are characterized by benchmark valuesfor transparency and conductivity. A good material for use in VPLETdevices should have a high transparency and a low sheet resistance.Currently the state of the art in transparent conductive electrodes isITO (Indium titanium oxide), which has an excellent combination of hightransparency and low sheet resistance. However, ITO is made of lowabundance elements, and thus increases device cost. There are manyfast-emerging transparent conductive materials like TCO (Transparentconductive oxide), PEDOTs (poly(3,4-ethylenedioxythiophene)) and otherhighly doped conductive polymers, carbon nanotubes and graphene. (See,e.g., C. Keplinger, et al., Science, vol. 341, pp. 984-987, 2013, thedisclosure of which is incorporated herein by reference.) As shown inTable 1, below, under the equivalent transmittance, the sheetresistances of most of these new materials are about 3 to 6 times higherthan that of ITO.

TABLE 1 Comparison of Current Transparent Conductive Materials MaterialType Typical Transparency Typical Conductivity ITO 85-95% 30-100 Ω/sqTCOs 80-93% 100-200 Ω/sq Conductive Polymers >85-90%  200-1000 Ω/sqMetal Grids <75% 3-100 Ω/sq Nano Silver Films >92% 100-300 Ω/sq CarbonNanotube Films 70-90% 100-1000 Ω/sq Graphene >90% 100 Ω/sq Nanowire (Ag)88-92% 10-300 Ω/sq

Accordingly, in many embodiments a transparent conductive electrodematerial is provided. In some embodiments such electrodes are formedfrom conductive random or patterned networks of nanowires (NW networks),including NW networks formed into supporting materials, such assupporting grids, meshes, plates, or other materials, such as holeycopper or holey graphite (graphene). In other embodiments, the NWnetworks may be formed by embedding a plurality of nanowires into anelastomeric material, such that an electrically conductive stretchableelectrode may be formed. In some embodiments, a Ag nanowire network isembedded in an elastomeric material to form a stretchable transparentelectrode with a transmittance of 80-85% and a sheet resistance of lessthan 100 Ω/sq, and in some embodiments from 10 to 25 Ω/sq.

In other embodiments, OLETs and VPLETs that incorporate such transparentconductive electrode materials, such as NW networks, for use, forexample, as supportive electrodes for the LETs and LEDs are provided.Embodiments of such electrode materials have now been discovered toexhibit equivalent or better figures of merit when compared tostate-of-the art materials, such as ITO. In particular, as summarized inTable 1 above, in many embodiments NW networks, for example, includinggrids, meshes, etc., may be formed that demonstrate broad sheetresistances from 10 Ω/sq to 300 Ω/sq, in some embodiments less than 100Ω/sq, and in some embodiments less than 15 Ω/sq; and hightransmittance >75%, and in some embodiments >90%.

Beyond transmittance and resistance another benchmark parameter that isoften disregarded is the porosity or surface coverage of the electrodematerial. Low surface coverage values permit light emitting materials todirectly contact the dielectric layer allowing for direct gatemodulation, which in turn allows for optimized charge-carrier balance,which is important for brightness, lifetime and external quantumefficiency of OLEDs. (See, e.g., C. V. Hoven, et al., Proceedings of theNational Academy of Sciences of the United States of America, vol. 105,pp. 12730-12735, 2008, the disclosure of which is incorporated herein byreference.) Accordingly, in many embodiments suitable highly transparentconductive electrode materials are formed into conductive porouselectrodes such that they have relatively low surface coverage <50%, insome cases <25%, in some cases <10%, and in some cases as low as <1%. Inembodiments, such conductive porous electrodes may be formed from arandom or patterned network of a plurality of metal or graphenenanowires, a nanowire metal mesh, a nanowire grid, a nanowire networkencased within an elastomeric material, and/or such nanowires supportedon holey copper or holey graphene.

OLETs thus formed with such porous conductive electrodes (e.g., formedof such NW network materials) offer ideal structures to control theinjection, transport and recombination of electron and hole, especiallyin both cathode and anode sides. These properties indicate thatembodiments of conductive porous electrodes formed from highlytransparent and porous materials, such as, for example, NW networks inaccordance with this disclosure, are superior materials for verticalOLETs when compared to other currently developed transparent conductivematerials. (Some exemplary benchmark properties are disclosed in Tables1 and 2.)

Although the above benchmarked properties have been established forconductive porous electrode materials formed from Ag NW nanowires, itshould be understood that any suitable conductive material capable ofbeing formed into conductive porous electrode materials having suitabletransmittance, resistivity and surface coverage properties may be usedin embodiments, including, for example, Ag, Au, Cu and graphene. In manyembodiments, the porous conductive electrodes are formed from NWnetworks formed from high aspect ratio nanowires having an aspect ratioof, for example, >1000. In some embodiments the NW networks are formedfrom a plurality of metallic nanowires, such as for example, Ag having adiameter of less than around 200 nm, in some embodiments less thanaround 35 nm, and a length of greater than around 1 μm, in someembodiments greater than 35 μm. Further, it should be understood thatthe conductive porous electrode materials may be formed of randomlydeposited NW (such as that shown in the SEM of an exemplary Ag NWconductive network presented in FIG. 2) or engineered NW that arearranged in grids or meshes or atop supportive materials such as holeycopper or holey graphene, and may also be produced in any shape anddimension suitable for use within a particular VPLET device. In someembodiments, the conductive porous electrodes formed of NW networks areproduced as thin sheets or strips that are sized to provide a supportiveconductive surface to a particular PLED layer within a VPLET device.

Finally, as will be described in greater detail below, many embodimentsare directed to OLETs and VPLETs that incorporate such conductive porouselectrode materials, such as NW networks, including Ag NW networks foruse, for example, as supportive electrodes for LET and LED devices.

LED and LET Devices

Although embodiments of porous conductive electrodes are describedabove, it should be understood that many embodiments are directed to LETand LED devices formed using such porous conductive electrode materialsas conductive elements within those devices. FIG. 3 provides a schematicof an exemplary VPLET in accordance with embodiments of the invention.As shown, the device generally a capacitor (C) and light emitting cells(LED) joined by a common electrode. Three electrodes are required in aVPLET, a common-source electrode, a gate electrode, and a drainelectrode, and associated contacts. A light emitting material, such as alight emitting polymer (LEP) forms an active layer between the sourceand drain electrodes, which in many embodiments also function as thecathode/anode of the LED. A high resistance dielectric layer is providedbetween the source and gate electrodes to complete the device.

Preferably the porous conductive electrodes (e.g., NW networkelectrodes) are formed with a surface coverage of no more than 50%, insome embodiments less than 25%, in some embodiments less than 10%, andin some further embodiments less than 1%, such that the dielectric canmake direct contact with the LEP to allow for direct gate modulation. Inmany embodiments, using such a vertical structure and the transparentconductive low surface coverage electrodes, VPLET devices may befabricate that exhibit a maximum external efficiency >5%,brightness >10,000 Cd/m² and full aperture at low power and low voltage,with improved lifetime and stability. A full list of exemplaryperformance metrics for devices constructed in accordance withembodiments of the invention are provided in Table 2, below.

TABLE 2 Performance Metrics for Ag NW Enabled VPLETs Figures of MeritsUnits Values Ag NW Concentration % >99 Ag NW Length μm  >1 Ink ViscositymPa · s <15 Ag NW Lifetime Days (free aggregation) >180  Ag NW thicknessμm <10 Ag NW Printing Density Tubes/μm² >30 Transmittance (Ag NW) % >90Resistivity (Ag NW) Ω/sq ~30 Ink Solution Type Au UT Dots Line WidthMinimum μm  10 Alignment Error ±μm  5 Resistivity Au Ω/sq  ~1 DielectricThickness μm  10 Gate Leakage Current pA/cm² >10 Polymer EmitterPFO/Super-yellow/ Supplier Merck MEH-PPV Mobility cm²/Vs 10⁻³-10⁻⁴On/Off Ratio Ratio  >10⁸ Switch Rate Hz >100  Supply Voltage V  <4Operating Voltage V  <1 Brightness Cd/m² 10000  Efficiency %  >5

Accordingly, in many embodiments vertical light emitting transistors areprovided that include a light emitting cell that includes a lightemitting layer formed of at least one light emitting material, a leastone capacitor that includes a dielectric layer formed of at least onedielectric material, and at least one substrate in supportive relationwith these elements. These elements (light emitting cells andcapacitors) are placed into appropriate conductive relation to drain,source and gate. In many embodiments, at least the electrode disposedbetween the light emitting layer and the dielectric layer is aconductive porous electrode, for example, formed of a transparentnetwork of a plurality of nanowires, wherein the nanowires have anaspect ratio of at least 1000, such that the porous electrode hassufficient open portions to exhibit a surface coverage of no greaterthan 50%, such that the dielectric layer makes direct contact with thelight emitting layer through the open portions of the porous electrode.

Although the above discussion focuses primarily on single gateelectrodes, as will be described VLET architectures incorporatingmultiple capacitors and gate electrodes are provided to form doublygated VLETs. Accordingly, in some embodiments the vertical lightemitting transistor includes at least two capacitors and two gateelectrodes, disposed between the drain electrode and a first gateelectrode and between the source electrode and a second gate electrode.

It should be understood that the materials and methods used to form therequired components of these VPLET devices may take any form suitablefor a desired application. For example, the substrate layers may beformed of either traditional rigid materials such as silicon wafer,glass, ITO, or flexible materials such as PET, Kapton, polyimides solong as the substrates allow for the deposition/growth of the necessaryfunctional layers of the devices (dielectric, LEP, transparentconductive electrodes, etc.).

Likewise, any suitable PLED may be used in association with embodimentsof the VPLET devices described herein. Some examples of suitable LEPsinclude, poly(1,4-phenylene vinylene) (PPV),Poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV),poly(1,4-phenylene) (PPP), polyfluorenes (PFO), nitrogen containingpolymers, such as, for example, poly(2,5-pyridine vinylene) orpoly(9-vinylcarbazole) (PVK), poly(thiophenes), and water-soluble LEPssuch as sulfonated PPV, PPP and PF; and light emitting small moleculesincluding organometallic Iridium complexes, small conjugated organicmolecules such as porphyrin and pentacene for example. It should also beunderstood that embodiments of VLETs do not have to use LEPs, and can beformed with other types of light emitting materials, includingsemiconductor nanowires such as may be formed of Si or GaAs, quantumwalls such as GaN, and conventional semiconductor materials across theentire wavelength spectrum, such as, for example, infrared emitters(GaAs), red emitters (AlGaAs & GaP), orange emitters (GaAsP & GaP),yellow emitters (AIGaInP & GaP), green emitters (GaN & GaP), blueemitters (ZnSe), violet emitters (InGaN), ultraviolet emitters (AIN),etc. And, these materials may be formed and/or embedded withinelastomeric materials such that they are flexible allowing for theformation of flexible VLETs and thus displays.

A variety of dielectric materials may also be used in embodiments, suchas for example, ionic gels formed of dielectric polymers and ionicliquids, such as, for example, polystyrene-co-poly(methylmethacrylate)-co-polystyrene (PS-PMMA-PS)/1-ethyl-3-methylimidazolium(trifluoromethylsulfonyl)imide (EMIM TFSI), BaTiO3/PMMA∥PEDOT/PSS,n-octadecylphosphonic acid SAM∥PEDOT/PSS, and CYTOP, and other printableor thermal vapor deposited dielectric materials such as dielectricpolymers including PMMA, Teflon, CYTOP, Nafion and the like; oxidesincluding, SiO₂, Al₂O₃, ZrO₂, HfO₂, TiO₂, and the like; inorganic saltsincluding, LiF, CsF, SrTiO₃, BaTiO₃ and the like; and nitrides includingSi₃N₄, etc.

A variety of additional layers might also be included in embodiments ofthe devices. For example, electron injection and transportation layersincorporating dipole molecules for PLEDs may also be included. Such thinlayers of such dipole molecules function by correcting the orientationof the dipole leading to an increase in the effective work function andthe improvement of electron injection in PLED devices. In particular, ithas been found that a spin-coated thin layer with 0.1% weight1-(3-hexadecylimidazolyl)-tris(1-imidazolyl)borate (C₁₆-Blm₄) inmethanol solution improved the performance of PLEDs. (See, e.g., H. P.Li, et al., Journal of the American Chemical Society, vol. 131, pp.8903-8912, 2009; and H. P. Li, et al., U.S. patent application Ser. No.12/592,761, 2009, the disclosures of which are incorporated herein byreference.)

Likewise, in some embodiments conjugated polyelectrolytes (CPE) may beused to improve the efficiency of PLEDs, polymer solar cells, andthin-film transistors. These improvements have been ascribed to theenhanced electron injection ability of CPE. (See, e.g., R. Yang, et al.,Journal of the American Chemical Society, vol. 128, pp. 14422-14423,2006; J. H. Seo, et al., Journal of the American Chemical Society, vol,133, pp. 8416-8419, 2011; and J. H. Seo, et al., Journal of the AmericanChemical Society, vol. 131, pp. 18220-18221, 2009, the disclosures ofeach of which are incorporated herein by reference.) Recently, it wasdemonstrated that by introducing a CPE layer between gold electrodes(Drain/Source) and light emitting polymers, the performance of PLETswere remarkably improved, again as a result of enhanced electroninjection again. (See, e.g., J. H. Seo, et al., Advanced FunctionalMaterials, vol. 21, pp. 3667-3672, 2011, the disclosure of which isincorporated herein by reference.) Alternatively or in addition, organicspecies, such as propionylethyleneimine-co-ethyleneimine (PEI-EI) may beintroduced to improve the electron injection of the LEPs.

Finally, for hole injection layers, polyanilines, PEDOTs andpolythiophene derivatives, as well as small molecules like cyano carbonssuch as, for example tetrafluoro-tetracyanoquinodimethane (F4-TCNQ),TCNQ, trichloroethylene (TCE), and others may be used. With gatemodulation, the induced increase of charge carriers (including electronor hole) can improve both charge injection and charge transportation.

In addition, although the above has assumed that the conductiveelectrodes would be formed of conductive porous electrode materials,such as, for example, NW network materials, it should be understood thatthe devices may also have other electrode types. For example, in someembodiments, one or more conductive porous source and drain electrodesmay be incorporated with gate electrodes formed from materials such as,for example, graphene sheets, ITO, Au, Al, Cu, Ni, Mo, Cr, Ag, metalnanowires, metal meshes, metal grids, holey copper and holey graphene,and conductive polymers such as,poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS).Likewise, although the electrode layers may be made of NW networks orother suitable materials, the contacts associated with those electrodelayers may be made of any suitable materials, such as, for example, Mo,Ni, Cr, Al, Cu, and Au.

Finally, although a generalized device structure has been described, itshould be understood that the materials and layers discussed above maybe combined in a variety of different configurations, including insingly gated VLETs (FIG. 4). For example, as shown in FIG. 4 a singlygated VLET in accordance with embodiments would generally include atleast drain, source and gate conductive porous electrode, such as a NWnetwork conductive electrodes surrounding a light emitting layer (suchas formed by a PLED), and a dielectric layer (such as an ionic gel)sandwiched between the light emitting layer and a further gate electrode(which could again be formed of a conductive porous electrode, such as,a NW network electrode). In these embodiments the singly-gated VLET maybe supported by any suitable material, such as, for example, a flexiblepolymer such as PET. A few exemplary singly gated VLET device materialarchitectures contemplated in accordance with embodiments would include:

-   -   a conductive porous electrode (e.g., NW network) on a Au (Al)        supported dielectrics layer (LiF, ionic gel) with ITO supported        polymer emitters to form NW enabled VPLETs gated by a        dielectrics layer (such as, e.g., LiF, ionic gel).    -   a conductive porous electrode (e.g., Ag NW network) on a silicon        wafer with ITO supported polymer emitters to form Ag NW enabled        VPLETs gated by SiO₂.    -   a conductive porous electrode (e.g., Ag NW network) on a silicon        wafer with a conductive porous electrode supported polymer        emitter to form NW enabled VPLETs gated by SiO₂.    -   a conductive porous electrode (e.g., Ag NW network) on flexible        substrates (PET) to form a NW electrode, a dielectrics layer        (ionic gel, LiF) atop a NW conductive porous electrode, a NW        conductive porous electrode again atop of the dielectrics layer,        and a NW|dielectrics layer|NW electrode with a NW conductive        porous electrode supported polymer emitter to form singly gated        NW enabled VPLETs on NW electrodes.

In addition, although the above has described conventional singly gatedarchitectures VLETs using novel combinations of materials in accordancewith some embodiments, other embodiments are directed to novelarchitectures allowing for doubly gated VLETs. Embodiments of suchdoubly gated VLETs are shown schematically in FIG. 5. As shown, thedoubly gated VLETS in accordance with embodiments include two gateelectrodes and two layers of dielectric material (such as ionic gels)sandwiching the drain and source electrodes and in turn the lightemitting layer (such as a PLED). An exemplary construction of such adoubly gated VLET might include, for example, a NW conductive porouselectrode on flexible substrates (such as, e.g., PET) to form NW gateelectrodes, then a dielectric layer (such as an ionic gel, LiF) atop ofa NW conductive porous drain electrode, NW conductive porous electrodesagain atop of a dielectric layer, and a NW|dielectric layer|NW electrodewith a NW electrode|dielectrics layer|NW supported polymer emitter toform doubly-gated NW enabled VPLETs on NW conductive porous electrodes.

Circuit diagrams comparing the construction of the singly and doublygated VLETS are provided in FIGS. 6a and 6b . As shown in FIG. 6a , in asingly gated VLET there is a single gated transistor interconnected withthe gate electrode along the data line. The source electrode is in turnconnected to the V_(DD), and the drain electrode is connected to the LElayer. The TFT turns on when a voltage pulse is applied to the gateelectrode. As a result, the signal voltage can be transmitted to thedrain, and subsequently, the light emitting layer (EL), which isconnected as load to the TFT.

While this is a well-known architecture, in some embodiments, a secondgate electrode and “gate line” is introduced into the circuits. As shownin FIG. 6b , in this embodiment, the light emitting layer isinterconnected to the drain electrode through this second gate electrode(labeled as gate bottom in FIG. 6b ). This doubly gated VLETarchitecture allows for the adjustment of the charge carrier from thetop and bottom electrodes, and for the control of the transportation,injection and recombination of charge carriers to reach charge carrierbalance for maximum efficiency and brightness. Moreover, whereas in asingly gated VLET an additional transistor is required to performswitching and scribing, in a doubly gated VLET a single device can serveboth functions simultaneously.

More detailed descriptions of these exemplary structures, theirconstruction and performance is provided in the Exemplary Embodimentsprovided below. However, although examples of specific structures andcombinations of materials and layers have been discussed, it should beunderstood that the various components described herein may be combinedand arranged in a variety of device architectures contemplated withinthe disclosed embodiments.

Methods for Manufacturing LET and LED Devices

Some embodiments are directed to methods of depositing NW conductivematerials for conductive porous electrodes to enable VPLETs and AMOLEDsgenerally. Although any suitable method of deposition may be used withthe current invention including, for example, additive manufacturing,molecular beam epitaxy (MBE) chemical vapor deposition (CVD), atomiclayer deposition (ALD), and plasma enhanced using low cost printing,spin-coating, spray coating methods, in many embodiments, printing, suchas aerosol jet printing, may be used to deposit one or more of thelayers of the LET and LED devices, including LEP inks as active lightemitting materials and NW inks as transparent conductive porouselectrodes of VPLETs on substrates. In particular, it has been foundthat printing methods, such as aerosol, roll to roll, screen and inkjet, for example, may be used to form NW networks, nanoparticles,polymer emitters, ionic gels, and conjugated polyelectrolytes that canbe used to improve PLET performance, and to produce NW porous conductiveelectrode supported polymer light emitting devices.

Printing is one of the most promising techniques for inexpensive largearea fabrication of plastic and nanomaterials electronics. Inparticular, aerosol-jet printing enables finer feature sizes thantraditional ink-jetting and screen printing technologies. Aerosol jetprinting utilizes an innovative direct-writing and aerodynamic focusingtechnology that produces electronics and physical structures withfeature sizes down to 10 microns line width and 20 nm thickness. Thesesystems can also print a wide variety of materials including conductivenanoparticle inks, screen printing pastes, polymers insulators,adhesives, etchants, and even biological matter onto various substrateslike non-planar surfaces. As a result, the range of possible componentsthat can be printed include transistors, integrated circuits,photovoltaics, LED's, displays and sensors. Also, direct aerosol-jetprinting simplifies the printing process as the material is deposited asa pre-defined pattern. Another aspect of current printing is itslow-cost, precise deposition, low environmental impact and large areafabrication due to the small number of process steps, small amount ofmaterials and high through-put. (See, e.g., J.-U. Park, et al., NatureMaterials, Vol. 6, pp. 782-789, 2007, the disclosure of which isincorporated herein by reference.)

In embodiments of the current invention printing system capable ofprinting over complex conformal surfaces, in deep trenches, and in highdense vertical interconnects for high performance of multi-chip packagesmay be used. One exemplary process utilizes a jet aerosol system such asthe 3-D Optomec Aerosol Jet™ printer.

One advantage of the described depositions methods is that the differentelectrical and electronic components can be printed on top of eachother, saving space and increasing reliability and sometimes they areall transparent. Accordingly, in some embodiments, deposition techniquesfor nonplanar surfaces, such as spin-coating or printing may be used todeposit the conductive porous NW network electrodes as well as coat LEPsor ionic gels atop of the NW network conductive porous electrodes toform integrated NW enabled VPLETs, which can be defined as displayproducts and components manufactured using deposition methods andelectrically functional materials, such as prinking with electricallyconductive inks. A flow chart of such a process is provided in FIG. 7.As shown, using a deposition technique in accordance with embodiments,it is possible, provided a suitable substrate is present, to deposit thesupportive porous electrodes (NW) and then directly deposit atop theseelectrodes the necessary LEP layers (for light emitting structures) ordielectric materials (for capacitor structures), and to repeat thisdeposition process as necessary to form singly or doubly gated devicesas described above.

In certain specific embodiments, aerosol jet printing can be utilized,where the aerosol jet printing is carried out using the followingparameters; ultrasonic atomization at a voltage that ranges from about20 to about 48 V, or pneumatic atomization with about 600 cubiccentimeters per minute atomizer flow to generate aerosol in a diameterof about 1 to 5 μm, a sheath gas flow of about 20 to 50 cubiccentimeters per minute, a carrier gas flow of about 10 to 20 cubiccentimeters per minute, a nozzle diameter of from about 50 to 350 μm,such as, for example, 60 μm, 100 μm, 150 μm, 200 μm, 250 μm, or 300 μm,and a fiducial management with overlay registration of from about 1 to 2μm. In certain embodiments, the carrier gas may be inert, includingultra-pure nitrogen, argon or combinations thereof. Utilizing such aprinting technique it is possible to manufacture single and doubly gatedVLETs that exhibit a maximum external efficiency >5%, brightness >10,000Cd/m² and full aperture at low power and low voltage, with improvedlifetime and stability.

EXEMPLARY EMBODIMENTS

Additional embodiments and features are set forth in part in theexemplary embodiment that follow, and in part will become apparent tothose skilled in the art upon examination of the specification or may belearned by the practice of the invention. None of the specificembodiments are proposed to limit the scope of the remaining portions ofthe specification and the drawings, and they are provided as exemplaryof the devices, methods and materials disclosed herein. In particular,although specific structures and particular combinations of materialsare recited, it should be understood that these are merely provided asexamples, and any suitable alternative architectures and materials maybe substituted.

Example 1 Spin Coating Ag NW Enabled VPLETs with Evaporated Dielectricsand Metal Gate Electrode on ITO

In one exemplary embodiment, shown schematically in FIG. 8, ITO was usedas an anode and the transmission layer from which light from the lightemitting cell may be emitted. A PEDOT/PSS (PH1000) was spin coated onthe ITO at 2,000 rpm to serve as a drain electrode (D). After annealingat 140° C. for 10 minutes, super yellow THF solution was spin-coated at5,000 rpm to form the PLED layer. On the top of super yellow layer(PLED), a 2 mg/mL Ag NW isopropanol solution was spin-coated at 2,000rpm and reapplied for at least >5 times to form a conductive sourceelectrode (S). Then a 200 nm thick silicon oxide was evaporated usinglow pressure chemical vapor deposition to form a dielectric layer. Ontop of the silicon oxide dielectric layer, 150 nm Al was sputterdeposited using an electrode mask to form a gate electrode (G). Theformed Ag NW enabled VPLETs were then characterized with a Keithley 4200Semiconductor Characterization System (SCS).

Example 2 Printing Ag NW Enabled VPLETs with ITO on Silicon Wafer

In one exemplary embodiment, printed Ag NW enabled VPLETs comprised ofITO as transparent porous conductive electrodes and light emittingpolymers as channel materials on silicon wafer are provided. Thisprocess includes printing a uniform Ag NW network on silicon wafer,LEPs, and laminating PEDOT/PSS coated ITO with printed LEPs to achieveprinted Ag NW enabled VPLETs with gate modulated brightness and externalefficiency higher than PLEDs. Using aerosol jet printing technologies itis possible to print Ag NW networks on any substrate layer-by-layerallowing for the printing of a Ag NW enabled VPLET on silicon wafers ofvarious SiO₂ thickness.

LEPs (blue: PFO, green: super-yellow, red: MEH-PPV) have been fabricatedinto typical OLETs using thermal evaporated gold electrodes (70 nmthickness) with 20 μm channel length and 1000 μm channel width, and showa maximum power conversion efficiency of 0.002-0.005 Cd/A, an externalquantum efficiency of 10-3, and a brightness <1000 Cd/m2 at supply anddriving voltages of 150 V. Zwitteronic electron injection layers havealso been developed for MEH-PPV OLEDs with a maximum power conversionefficiency of 1.1 Cd/A, and a brightness >12,000 Cd/m2. (See, e.g., H.P. Li, Y. Xu, 2009; and H. P. Li, Y. Xu, and G. C. Bazan, 2009, citedabove.) VPLETs were also fabricated with LiF as a dielectric layerbetween two Al electrodes showing gate modulation, but no detailedinformation was provided on the VPLET's characterization. (See, e.g., Z.Xu, S. H. Li, L. Ma, G. Li and Y. Yang 2007, cited above.)

As shown in FIG. 9a , in this exemplary embodiment, a printed Ag NWenabled VPLET is proposed comprised of printed Ag NW networks as aporous conductive Source electrode, LEPs (blue: PFO, green: superyellow, red: MEH-PPV) as channel materials (PLED), PEDOT/PSS coated ITOas a Drain electrode, and SiO₂ as the dielectric layer for gatemodulation. Using aerosol jet printing, uniform and thin-layer Ag NWnetworks and 200 nm LEPs (blue: PFO, green: super-yellow, red: MEH-PPV)may be printed in various substrates. The sheet resistance of printed AgNW networks is about 15 Ω/sq. Its SEM image is shown in FIG. 9b ,exhibiting great uniformity. The porous structure of Ag NW networkspermits the direct contact between LEPs and SiO₂ dielectric layer. FIG.9b also shows a photograph of a printed MEH-PPV polymer with a uniformsurface that is essential for organic devices. In this embodiment, bylaminating PEDOT/PSS coated ITO with printed MEH-PPV on Silicon wafer,Ag NW enabled VPLETs with an ITO electrode on silicon wafer may befabricated. Alternatively, printed Ag NW networks can be laminated withMEH-PPV fabricated on ITO by slight pressing and thermal treatment. Toimprove the performance of Ag NW enabled VPLETs, it is also possible tocoat the SiO₂ layer with octyltriethoxysiliane (OTS) to improve the gatemodulation and treating Ag NW network with CPE or polyethylimine (PEI)to enhance electron injection. The printed Ag NW enabled VPLETs may becharacterized in a glovebox using a Keithley 4200 SemiconductorCharacterization System (SCS). Using such process it is possible tofabricate Ag NW enabled VPLETs with characteristics outperforming thoseof PLEDs, with low supply and driving voltages, and full aperture ratio.

Example 3 Printing Ag NW Enabled VPLETs with Ag NW Electrode on Si Wafer

In another embodiment, an aerosol jet printing technique is provided forprinting Ag NW on flexible plastics such as polyethylene terephthalate(PET) to fabricate flexible transparent porous conductive electrodes.Printing uniform Ag NW on PET allows one to replace ITO. In a particularembodiment, printed Ag NW porous conductive electrodes will bespin-coated with PEDOT/PSS and then will be laminated with printed LEPson Ag NW covered silicon wafer to achieve Ag NW enabled VPLETs with a AgNW electrode on silicon wafer. Alternatively, it is possible to use theAg NW stretchable electrode fabrication method described above. In onesuch method Ag NW is printed on the top of a substrate such as glass,and then in-situ polymerization is performed atop the Ag NW networks.The peeled-off polymer films are porous, transparent and conductive withthe embedded Ag NWs.

As shown in FIGS. 10a and 10b , in this embodiment Ag NW networks areprinted on PET plastic. The sheet resistance and transmittance ofprinted Ag NW electrodes are 15 Ω/sq and 90%, respectively. The SEMimage (FIG. 10b ) shows good uniformity and consistency. Following thelamination method in Example 1, the printed Ag NW porous conductiveelectrodes can be laminated with printed LEPs on Ag NW network coveredsilicon wafer with the assistance of pressure and heating. LEPs can alsobe printed on Ag NW porous conductive electrodes and laminated withprinted Ag NW on silicon wafer. Also, on the top of the printed LEPs onAg NW covered silicon wafer, Ag NWs can be further printed to fabricatefully printed Ag NW enabled VPLETs with Ag NW porous conductiveelectrode on silicon wafer, as shown schematically in FIG. 10a . Toachieve better performance, the silicon wafer may be treated with OTS,and the printed silver NW electrodes may be treated with CPE or PEI. Asbefore, the fabricated devices may be characterized with a Keithley 4200SCS in a glovebox having a N₂ atmosphere. The characteristics of Ag NWenabled VPLETs with Ag NW porous conductive electrode on silicon waferare anticipated to be similar to those of Ag NW enabled VPLETs with ITOelectrode on silicon wafer, but much better than those of PLEDs with AgNW porous conductive electrodes. In particular, it is anticipated thatthe Ag NW porous conductive electrodes will have transmittance andconductivity greater than ITO.

Example 4 Printing Singly-Gated Ag NW Enabled VPLETs with Ag NWElectrode on Ionic Gel Coated Ag NW Substrate

In yet another exemplary embodiment, methods for replacing silicon waferwith Ag NW porous conductive electrodes and a PS-PMMA-PS/EMIM TFSI ionicgel dielectric layer are disclosed. Of particular note in thisembodiment is the requirement to print a second layer of Ag NW networkson the top of PS-PMMA-PS/EMIM TFSI ionic gel. One of the critical issuesin such a device is the problem of short currents between the two layersof Ag NW networks. One method of addressing this issue is to embed Ag NWnetworks with PS-PMMA-PS/EMIM TFSI ionic gel, and then laminate ionicgel imbedded Ag NW networks with Ag NW electrodes (Ag NW networksimbedded in polymer films). LEPs will then be printed on the fabricatedAg NW networks atop the PS-PMMA-PS/EMIM TFSI ionic gel on Ag NWelectrode, and then be laminated with PEDOT/PSS coated Ag NW electrodeto form printed singly-gated Ag NW enabled VPLETs with Ag NW electrodeon PS-PMMA-PS/EMIM TFSI ionic gel coated Ag NW substrate.

As shown in FIG. 11, in this embodiment a printing technique is used toprint a PS-PMMA-PS/EMIM TFSI ionic gel on the top of printed Ag NWporous conductive electrodes and further print Ag NW networks tofabricate Ag NW networks sandwiched ionic gel dielectric layers. LEPsmay then be printed on the Ag NW porous conductive electrodes or on theAg NW porous conductive electrodes sandwiched ionic gel and laminatedwith Ag NW electrodes sandwiched ionic gel or Ag NW electrodes toachieve singly gated Ag NW enabled VPLETs with a Ag NW electrode onionic gel coated Ag NW substrate.

As discussed above, the major challenge to such a printed Ag NW porousconductive electrode sandwiched PS-PMMA-PS/EMIM TFSI ionic gel device iscross connection between the two Ag NW porous conductive electrodes, andalso in controlling the thickness of PS-PMMA-PS/EMIM TFSI ionic gel.Material diffusion, printing parameter optimization, baking thermalbudgets and material stability parameters are key concerns. To addressthis challenge it is possible to use Ag NW electrodes in which the Ag NWnetworks are embedded in in-situ polymerized transparent plastics. (See,e.g., J. Liang, et al., Nature Photonics, vol. 7, pp. 817-824, 2013, thedisclosure of which is incorporated herein by reference.) In some suchembodiments, Ag NW networks will be printed on glass slides, and will becoated with the mixture solution of a siliconized urethane acrylateoligomer (UA) and an ethoxylated bisphenol A dimethacrylate (EBA). Themixture solution will be in-situ polymerized to form transparent films.After peeling the film off glass slide, the transparent conductive Ag NWelectrode is characterized to be of 90% transmittance and 15 Ω/sq sheetresistance with Ag NW networks embedded in the film (FIG. 8). UA and EBAare chosen for the high transparency and excellent stretchability of thehomopolymer of UA and the good bonding force between the homopolymer ofEBA and AgNWs, although other suitable materials may also be used.

FIG. 11 exhibits the obtained transparent conductive Ag NW PUA film. Asshown, a PS-PMMA-PS/EMIM TFSI ionic gel will be printed on thefabricated Ag NW transparent conductive film (TCF) to act as thedielectric layer. PS-PMMA-PS/EMIM TFSI has demonstrated high conductancefrom P3HT (poly(3-hexylthiophene)) TFTs for driving OLEDs at 4 V supportvoltage and sub-1 V driving voltage. (See, e.g., J. H. Cho, et al.,Nature Materials, vol. 7, pp. 900-906, 2008; Y. Xia, et al., AdvancedFunctional Materials, vol. 20, pp. 587-594, 2010; and D. Braga, et al.,Advanced Functional Materials, vol. 22, pp. 1623-1631, 2012, thedisclosures of which are incorporated herein by reference.) The ionicliquid can penetrate into the active layer, which will behave as a lightemitting electrochemical cell (LEC). (See, e.g., Q. Pei, et al.,Science, vol. 269, pp. 1086-1088, 1995, the disclosure of which isincorporated herein by reference.) Printed PS-PMMA-PS/EMIM TFSI ionicgel on Ag NW TCF will be stacked on printed Ag NW networks on a glassslide. After drying PS-PMMA-PS/EMIM TFSI ionic gel, Ag NW TCF will bepeeled off from the glass slide to obtain a Ag NW electrodes sandwichedionic gel. PEDOT/PSS will be printed on another fabricated Ag NWtransparent conductive film. After drying a PEDOT/PSS coated Ag NW filmunder vacuum, 200 nm thick LEPs (blue: PFO, green: super-yellow, red:MEH-PPV) will be printed to serve as the active layers.

By laminating a Ag NW electrodes sandwiched ionic gel with printed LEPson a Ag NW electrode, singly-gated Ag NW enabled VPLETs with Ag NWelectrode on ionic gel covered Ag NW substrate will be achieved (asshown in the process diagram of FIG. 11). The obtained devices may becharacterized in a glovebox with a Keithley 4200 SCS, and areanticipated to have a maximum external efficiency >5%, abrightness >10,000 Cd/m2, and a full aperture ratio at 4 V supplyvoltage and sub-1 V driving voltage. Further, with >90% transmittance inthe Ag NW electrodes, the light can emit from both the top and bottom ofthe device with full aperture.

Example 5 Printing Doubly-Gated Ag NW Enabled VPLETs with Ag NWElectrode on Ionic Gel Coated Ag NW Substrate

In still yet another embodiment, method and devices for printeddoubly-gated Ag NW enabled VPLETs with Ag NW electrode on ionic gelgated Ag NW substrate are provided. In particular embodiments laminatingAg NW embedded PS-PMMA-PS/EMIM TFSI ionic gel atop of Ag NW electrodewith printed LEPs on Ag NW embedded PS-PMMA-PS/EMIM TFSI ionic gel atopof Ag NW substrate are used to achieve doubly-gated Ag NW enabled VPLETswith a Ag NW electrode on ionic gel coated Ag NW substrate. Many of thesame technical challenges arise as with Example 4. The advantage isthat, in addition to low power, low voltage and high aperture ratioadvantages, with doubly-gated Ag NW enabled VPLETs by PS-PMMA-PS/EMIMTFSI ionic gel, the charge carrier can be controlled with the two gateelectrodes to reach the best performance without the requirement ofadding additional transistors.

As shown in FIG. 12, embodiments of these doubly gated devices are basedon printing Ag NW electrode sandwiched PS-PMMA-PS/EMIM TFSI ionic gels,such as are describe in detail in Example 4, above. In some embodiments,on one Ag NW electrodes sandwiched PS-PMMA-PS/EMIM TFSI ionic gel, 200nm thick LEPs are also printed and laminated with another Ag NWelectrodes sandwiched PS-PMMA-PS/EMIM TFSI ionic gel to obtaindoubly-gated Ag NW enabled VPLETs with Ag NW electrodes on ionic gelcoated Ag NW substrate (as shown in FIG. 12). By adjusting the chargecarrier from the top and bottom Ag NW electrodes, the transportation,injection and recombination of charge carriers can be controlled toreach the charge carrier balance for the maximum efficiency andbrightness. As in the singly-gated Ag NW enabled VPLETs, doubly-gated AgNW enabled VPLETs are expected to have >5% external efficiency, >10,000Cd/m2 brightness, and a full aperture ratio at 4V supply voltage andsub-1 driving voltage when characterized in a glovebox with a Keithley4200 SCS.

Example 6 VLET Displays

Finally, although the above exemplary embodiments and discussion hasfocused on methods, architectures and structures for individual devices,it will be understood that the same architectures and structures may becombined as pixels into a VLET display device as shown schematically inFIG. 13. In such an embodiment, a plurality of VLETs as described hereinmay be combined and interconnected as is well-known by those skilled inthe art, such as by electronically coupling the VLETs into addressingelectrode lines, to form a TFT-backplane for a display, such as anAMOLED display.

SUMMARY

The market desire for high quality, large-size and low-cost displays aredriving a need for improved display products. The current disclosureprovides embodiments describing, among other things:

-   -   Simple, scalable, reproducible, and environmental robust        techniques for printing porous conductive electrodes,        semiconducting channel (LEPs), and organic gating materials;    -   Printed porous conductive electrode enabled vertical polymer        light emitting transistors on silicon wafer with high        conductance at lower power and low voltage;    -   Printed transparent porous conductive electrodes from NW films;    -   Singly-gated or doubly-gated porous conductive electrode enabled        vertical polymer light emitting transistors with porous        conductive electrodes on PS-PMMA-PS/EMIM TFSI ionic gel coated        NW substrate to achieve >5% external efficiency, 10,000 Cd/m2        brightness and full aperture at 4 V supply voltage and sub 1 V        driving voltage.

The vertical structure, porous conductive electrodes (such as, forexample, Ag NW networks), and high capacitance of dielectric materialsdescribed herein remarkably improve the performances of VPLETs such thatthey are superior to current AMOLED combinations. Moreover, embodimentsdirected to spin-coating, aerosol jet printing technologies to implementprinting LEPs, porous conductive electrodes, and dielectric materialsfor VPLET devices utilize the innovative direct-write and aerodynamicfocusing technology to produce high resolution features down to 10 μm onnon-planar substrates without the need for the masks or other high costtechniques. The spin-coating/printing technologies also reduce themanufacturing cost and environmental impact, and increases fabricationsize with high throughput. Printed porous conductive electrodes (e.g.,Ag NW) enabled VPLETs are expected to operate at low supply voltage(<4V) and sub-1V driving voltage with negligible signal delay (<50 μs),to generate enormous contrast ratio and power savings. In addition,printed porous conductive electrodes (e.g., Ag NW) enabled VPLETs onflexible substrates can expedite a shift to printed flexible devices.

DOCTRINE OF EQUIVALENTS

Having described several embodiments, it will be recognized by thoseskilled in the art that various modifications, alternativeconstructions, and equivalents may be used without departing from thespirit of the invention. Additionally, a number of well-known processesand elements have not been described in order to avoid unnecessarilyobscuring the present invention. Accordingly, the above descriptionshould not be taken as limiting the scope of the invention.

Those skilled in the art will appreciate that the presently disclosedembodiments teach by way of example and not by limitation. Therefore,the matter contained in the above description or shown in theaccompanying drawings should be interpreted as illustrative and not in alimiting sense. The following claims are intended to cover all genericand specific features described herein, as well as all statements of thescope of the present method and system, which, as a matter of language,might be said to fall there between.

What is claimed is:
 1. A method to form vertical light emittingtransistors, comprising: forming a light emitting layer formed of atleast one light emitting material, the light emitting layer having firstand second sides; forming at least one of a conductive drain electrodeand a conductive source electrode in conductive relation with at leastone side of said light emitting layer; forming a least one capacitorcomprised of a dielectric layer formed of at least one dielectricmaterial, the at least one dielectric layer having first and secondsides in conductive relation to one of either the conductive source ordrain electrodes, and a conductive gate electrode; encapsulating theformed layers using one of either a glass or barrier film; and whereinat least the electrode disposed between the light emitting layer and thedielectric layer is a conductive porous electrode that has sufficientopen portions to exhibit a surface coverage of no greater than 50%, suchthat the dielectric layer makes direct contact with the light emittinglayer through the open portions of the conductive porous electrode. 2.The method of claim 1, comprising one of the following combination ofsteps: disposing the light emitting material atop a PEDOT coated ITOsubstrate, disposing one of either the drain or source electrode atopthe light emitting layer, disposing the dielectric layer atop the one ofeither the drain or source electrode present atop the light emittinglayer, and disposing the gate electrode atop the dielectric layer;disposing a first dielectric layer atop a first gate electrode,disposing one of either the drain or source electrode atop the firstdielectric layer, disposing the light emitting layer atop the one ofeither the drain or source electrode disposed atop the first dielectriclayer, disposing one of either the drain or source electrode atop thelight emitting layer; disposing a second dielectric layer over the oneof either the drain or source electrode disposed atop the light emittinglayer, and disposing a second gate electrode atop the second dielectriclayer; disposing the dielectric layer atop the gate electrode, disposingone of either the drain or source electrode atop the dielectric layer,and disposing the light emitting layer atop a PEDOT coated ITO substratethat is further laminated with the one of either the drain or sourceelectrode which is disposed atop the dielectric layer; and disposing thedielectric layer atop the gate electrode, disposing one of either thedrain or source electrode disposed atop the dielectric layer, disposingthe light emitting layer atop the one of either the drain or sourceelectrode disposed atop the dielectric layer, and disposing the lightemitting layer that is laminated with the one of either the drain orsource electrode not disposed atop the dielectric layer.
 3. The methodof claim 1, wherein the light emitting layer is formed by a depositionprocess selected from plasma enhanced chemical vapor deposition, lowpressure chemical vapor deposition, molecular beam epitaxy,spin-coating, spray coating, aerosol jet printing, inkjet printing,screen printing, gravure printing, and flexograph printing.
 4. Themethod of claim 3, further comprising thermal annealing the lightemitting material in an inert atmosphere at a temperature range of from60 to 140° C. for a duration of about 10 to 60 minutes.
 5. The method ofclaim 3, wherein the light emitting layer is a conjugated polymer formedby spin-coating the conjugated polymer in an organic solution having aconcentration of 1 to 10 mg/mL with a speed range from 800 to 5000 rpmfollowed by thermal annealing the conjugated polymer in an inertatmosphere at a temperature range of from 60 to 140° C. for a durationof about 10 to 60 minutes.
 6. The method of claim 3, wherein the lightemitting layer is a conjugated polymer formed by aerosol jet printingthe conjugated polymer in an organic solution using a sheath gas flow ofabout 20 to 50 cubic centimeters per minute, a carrier gas flow of about10 to 20 cubic centimeters per minute, a nozzle diameter of from about60 to 300 μm, and an atomization selected from ultrasonic atomization ata voltage that ranges from about 20 to 48 V, and pneumatic atomizationwith an atomizer flow of about 600 cubic centimeters per minute, togenerate an aerosol having a diameter of about 1 to 5 μm.
 7. The methodof claim 1, wherein at least one of the source, drain, and gateelectrodes comprise a metal selected from Al, Au, Ag, Cu, Ni, Cr, Mo,and their combination formed by plasma enhanced chemical vapordeposition or low pressure chemical vapor deposition.
 8. The method ofclaim 1, wherein the at least one conductive porous electrode is formedfrom a plurality of nanowires formed into one of the group of a randomor patterned network of a plurality of metal or graphene nanowires, ananowire metal mesh, a nanowire grid, a nanowire network encased withinan elastomeric material, holey copper and holey graphene.
 9. The methodof claim 8, wherein the conductive porous electrode comprises a silvernanowire network with a surface coverage less than 10%, a sheetresistance less than 100 Ω/sq, and a transmission greater than 75%, andis formed by a process selected from spin-coating, spray coating,aerosol jet printing, inkjet printing, screen printing, gravureprinting, and flexograph printing the silver nanowire network followedby thermal annealing the silver nanowire network in an inert environmentat a temperature range of from about 60 to 160° C. for a duration ofabout 10 to 60 minutes, and washing the silver nanowire network withisopropanol.
 10. The method of claim 9, wherein the conductive porouselectrode is formed by spin-coating a Ag nanowire isopropanol solutionwith a concentration range of from about 0.1 mg/mL to 10 mg/mL with aspinning speed range of from about 800 to 5000 rpm, and repeating saidspin-coating from about 1 to 20 times.